Towell Miter – secondary dovetails and Bridge mortises

This entry will cover the layout and cutting/filing work for the bridge mortises and secondary dovetails on the sidewall stock. First, the bridge work.

Bridge mortise layout

The layout of the mortises for the bridge are absolutely critical to the final outcome of the plane. Variations in layout from one side to the other will at best require some serious finagling to correct. At worst, they will make the sidewall unusable and require starting over from scratch.

To ensure symmetry, I produce a simple template indexed precisely to both the top and the front edge markings on the sidewall stock.

This is a one-time-use template, so I’ll make it out of graph paper. To lay out the mortises, first I need to establish the angle the bridge will lie in with respect to the sole. The blade will be bedded at 22.5 degrees, and the bridge needs to be at an angle a bit higher than that to accommodate the wedge. I like to use a wedge angle of about 8 degrees, so I’ll use 30.5 degrees as the bridge-to-sole angle. To get the vertical placement correct, I want to register the bridge of the bed, leaving enough room for both the blade and some wedge thickness.The bed location is easily drawn in, referencing the working drawings for the plane. From those, I establish a baseline for the wedge by marking out a 30.5-degree line directly from the rear of the mouth, where the bed meets the sole – from this line I’ll establish the amount of offset I’ll need to accomodate the blade and wedge. You can see the baseline in the photo below – it’s the dotted line between the bed and bridge. I’m planning a 3/16″ blade, so I am using 1/4″ of offset from the ‘baseline’, which should allow for a reasonable wedge thickness as well. Please note that the offset should be marked off perpendicular to the bridge baseline – not perpendicular to the sole.

The bridge itself is 5/16″ thick, so I lay out a second line 1/4″ above the bridge line – this line is 1/16″ shy of the top of the bridge to allow for a shoulder at the top of each tenon – then I place the mortises within the space I’ve defined. I’m planning to use a bridge that is about 1-1/2″ long, so I lay out my mortises 5/16″ wide with 1/2″ space between them.

Here is the paper layout:

Now, I cut this layout out for use as my template, paying special attention to the index lines, which are the front and top edges of the sidewalls. These are what will ensure I’m laying the mortises exactly symmetrically. First on the ‘left’ portion of the sidewall :

Then I flip the template front-for-back and mark the right side of the stock:

At this point, as a self-check I scribe a pair of 30.5-degree lines on the stock to coincide with the top and bottom of the mortises. These are a good double-check to ensure the template markings are set in the proper plane.

The mortise layout looks good, so it’s on to the secondary dovetails.

Laying out and filing secondary dovetailsOne of the interesting things about the Towell miter is the appearance of the dovetails on the sidewalls.

If you look closely, you’ll see that each dovetail is actually only ‘flared’ in the lower 1/8″ or so – the top 3/16″ of each joint is perpendicular to the sole. This is the first time I’ve seen this on a plane – my best guess is that Towell used this technique because it greatly reduces the size of the gaps at the top of each joint, which must be peined shut later on. With such a large sole, a full dovetail would be quite a chore to close up, and is really not necessary for strength purposes. I think this is somewhat of an aesthetic compromise, but it is a very interesting feature and I’m going to emulate it here; since the plane is steel-on-steel, the dovetails are not particularly visible after lapping anyway, and it strikes me as an interesting technique to try out.

The layout for these is a bit more work than conventional full-depth secondary dovetails, but the basic premise is the same. I scribe in a second ‘baseline’ 3/16″ from the bottom of each tail (1/16″ of this is my peining excess), and mark in the dovetailed portion to this line. Above the line, I mark vertical lines to the top of each tail.

The measurements for this feature are selected in part to fit the tooling I have – my smallest pillar file is about 3/16″ wide, which will fit the ‘flat’ portion that needs to be filed perfectly. First, I file the flared portion of the joint:

Then I file each flat, trying to keep the two surfaces matched up properly:

And that’s the process. Conventional dovetails are done the same way, but the flared portion continues all the way to the top of the joint – making the process a fair bit simpler and faster.

Cutting and filing bridge mortises

Next, I drill and file open the bridge mortises. The trick here is to drill out as much as you can without going over the lines, then file to the lines. I will sometimes use a jeweler’s saw to help remove some of the waste, but in this case I managed to get a lot of waste out with the drill. If you don’t own one, I highly recommend investing in a spotting drill – I have a 1/4″ one that cost me $5 or so, and has lasted me two years without sharpening. Spotting drills are very short, with a solid round shank and a single flute, all of which makes them very stable, and nearly immune to ‘walking’ and distortion. Ideally, you want one with a tip angle to match your standard drills, so you can start a hole and have solid registration of a twist drill when you switch over. I center punch where I want my hole, start a shallow hole with the spotting drill and then finish the hole with a standard twist bit – with this technique, I find I have a remarkable degree of precision in drilling, which lets me remove as much waste as possible before I switch to files.

Once I’ve filed to the scribe marks, the last step is to file a taper on each edge of the outside to give some grip to the tenons when I pein them in place.

The last operation before bending is to cut the dovetails for the front wall – but for now I decide to do some cleanup and remove the layout fluid. I deepen the scribe marks for the centerline and each end, as I’ll need these when I do the bend and cut the front plate dovetails.

That strikes me as enough for one night, so I decide to break at this point. Here is the sidewall stock as it is now:

Next comes the front plate, dovetails, bending, and the start of work on the two-piece sole. It may be a few days before I get everything done and written up for the next installment, but it shouldn’t take too long. Stay tuned…

Comments

I am really enjoying this series of posts. May I suggest that it would be worthwhile, when you have finished transferring them into a downloadable form (esp a pdf) so that those of us who also want to try to build planes like this in the future can keep a copy for themselves on their hard disk (unless of course you plan to publish these posts in book form, in which case, put me down for a copy!).

Steve #1 – Eventually, it is my intention to add some more content to the umbrella site for this blog, including photos and probably some additional tutorials on some different stuff. The issue, always, is time.

When I was in Australia I met Colen Clenton and purchased one of his marnkig gauges. It is a work of art and I’m always so careful when I use it because I’m afraid to mess it up. I need to get over that because its a tool that is built to be used. I agree its a very expensive tool but I figure its something I’ll have for the rest of my life so why not. I also want to take this opportunity to thank you for the great time I had taking the 6 day class and for all I learned. You taught me so much and I can’t thank you enough! ThanksJeff Smith